The PTH/PTHrP receptor works through at least two distinct signaling pathways by activation both Gs and the Gq family. Previous studies have provided suggestive but inconclusive evidence concerning the linkage of specific second message pathways and particular distal actions of PTH and PTHrP in kidney and bone. The goal of this proposal is to establish the physiologic roles of specific second messenger pathways in PTH and PTHrP action in vivo. Lines of mice have been established in which the normal PTH/PTHrP receptor gene is replaced with a mutant receptor gene. In this mutant receptor, the alteration of four adjacent residues leads to dramatic blunting of activation of Gq without affecting the activation of Gs. Analysis of the effects of inactivating the Gq pathway on tissue responsiveness in intact animals will establish the physiologic roles of the Gq pathway in the actions of PTH and PTHrP. The results may help in the design of ways to control hypercalcemia and to use the anabolic actions of PTH to treat osteoporosis. Specific Aim I of the proposal will be to determine the role of PLC signaling by the PTH/PTHrP receptor in the growth plate, following up the discovery that these mice have abnormal growth plates. Specific Aim II will evaluate the effects of the mutation on the bone responses to PTH. Particular focus will be placed on the regulation of the anabolic effect of PTH and the stimulation by PTH of bone resorption. Specific Aim III will study the effects of the mutation on the renal responses to PTH, since calcium and phosphorus homeostasis are disrupted when these mice are challenged with a low calcium diet. Particular focus will be placed on the regulation of sodium-phosphate co-transport and of 25-hydroxyvitamin D 1alpha-hydroxylation.